In the subscriber loop motion of telecommunication systems, shielded-pair cable is the predominant medium utilized for signal propagation. The cable generally comprises numerous conductor pairs encompassed within a nonconductive sheath; the sheath also contains a continuous metallic shield so the cable may be grounded periodically to mitigate interference. The conductors of a pair are generally known as the tip and ring conductors. The tip and ring, together with circuit ground, constitute a three-wire transmission line utilized for signaling and testing purposes. Typically each pair connects customer equipment to a switching point, usually a central office, or groupings of pairs called trunks connect switching points.
During the course of usage of telephone equipment by a customer, situations occur wherein a telephone receiver is inadvertently removed from the switchhook, a condition referred to as a ROH (receiver-off-hook) fault. As a service to the customer, in certain central offices, an audible signal is temporarily connected at the central office to the pair serving the customer so as to alert the customer of a ROH. This signal is placed on the loop after loop current flows and no digits are dialed within about sixty seconds. After the audible signal is removed, the pair is connected to central office circuitry designated a permanent signal holding network. It is also well-known that other conditions, such as low tip-ring resistance caused by a fault, are also interpreted as a ROH condition by the central office.
A typical holding network applies a resistance (which may be zero) to the tip conductor, a battery in series with a resistance to the ring conductor and, usually, a 500 Hz tone ring-to-ground. The characteristics of the holding network for each particular central office type are referred to as its signature.
Oftentimes it is necessary to test a subscriber loop to determine if faults exist or if preventive maintenance is required. These tests are usually performed over a test trunk pair from a remote location having a controller and associated test equipment. When a loop is accessed by a very short test trunk (essentially zero resistance), then the signature of the holding circuit closely matches the actual measurements on the three-port network comprising the tip and ring conductors and ground. This affords a simple detection of a permanent signal condition so that additional tests may be planned and performed.
However, if a test trunk is electrically long, then the three-port equivalent circuit derived from measurements at the test point may depart substantially from the signature. In fact, the terminating network characteristics may be so completely obfuscated that the network is not identifiable. This distortion may result in improper fault diagnosis, incorrect selection of follow-up measurement; strategies, or even false dispatches of repair forces.
One technique to counteract this deleterious effect is to utilize calibrated trunks. For instance, the trunk resistance may be measured periodically and the equivalent circuit interpreted in view of this known resistance. However, for certain holding networks, small errors in the trunk resistance due, for example, to measurement inaccuracies or changing ambient conditions, are magnified so that the results are still misinterpreted.
The foregoing focused on holding networks placed on a loop in the central office and measurements over an electrically long turnk circuit. A similar interpretation problem exists whenever loop measurements are made at the central office to detect conductor-to-ground conditions along the loop. For instance, a coin telephone has unique three-port network characteristics in its different operating modes. In these situations, the unknown resistance is the loop resistance between the central office and the network connection point. It is not practical to calibrate the loop so direct interpretation of results is extremely difficult.